Issue/17 consistency

changed the models for strong consistency, which means every entity has a direct parent, except for two root models (User and Club)

_implication_: the resources should be adressed with the whole ancestor path (i.e. /trainers/1/clients/2/phases/3/circuit/4/exercises/5, or /clubs/1/trainers/)

transfered the models to use the NDB api from google-appengine instead of DB (always stay cutting edge!)

also added some documentation for the API (with examples)
The issue is that trainers can change clubs, so they shoudn't be tied to one specifically. They have a reference field for the club they're part of, and can easily be changed (remember that the 'parent' of an entity cannot be changed).
But you're right, there will be only eventual consistency when managing trainers.